My proposed research and training program aims to improve the mobility and quality of life of veterans with transtibial (TT) amputation through the development of a novel biarticular prosthesis, and to train me in neuromuscular biomechanics, prosthetic engineering, and gait simulations through the guidance and support of a strong multidisciplinary team of mentors, so that I can become a successful independent research scientist. My long-term career goal is to design, develop and evaluate novel prosthetic, orthotic and assistive devices for people with reduced mobility. Consistent with this goal, I am proposing to design, develop and evaluate a novel biarticular prosthesis that addresses some of the limitations of current lower limb prostheses. Approximately one million people in the US are living with lower limb loss. An important cause of mobility impairment in this population is the loss of the ankle plantar flexors since these muscles are the primary contributors to body support, forward propulsion and swing leg initiation during normal walking. The biarticular gastrocnemius (GAS) has a role distinct from that of the soleus; it provides the dominant source of energy to power the leg into swing phase. Without the GAS, compensatory changes of the remaining leg muscles are necessary, including an increase in the concentric iliopsoas force to accelerate the leg into swing phase. Current prosthetic limbs cannot replicate the physiologic function of the ankle plantar flexors. When walking with conventional prosthetic foot-ankle systems, TT amputees show limited push-off power, and they use adaptive gait strategies, such as increased hip flexor work on the prosthetic side during pre-swing, to compensate for the lack of a biarticular GAS. These compensations are thought to contribute to the increased metabolic cost of amputee walking. To reduce the gait compensations typical of TT amputees, I am proposing to develop a novel biarticular prosthesis (BP) with a clutched spring that spans the ankle and knee joint. The BP will have the ability to generate net positive ankle joint work and to provide energy to power the leg into swing. I will use musculoskeletal modeling and simulations of TT amputee gait to optimize the design of the BP, and I will use gait analysis to evaluate the effect of the BP on gait. The BP has the potential to reduce gait compensations and improve functional mobility in veterans with TT amputation. The proposal has three specific aims: Aim 1: Using simulation, determine subject-specific optimal BP parameters. Aim 2: Demonstrate that the TT amputee gait compensations typically seen with conventional passive- elastic prostheses are reduced with the subject-specific optimal BP across a range of walking speeds. Aim 3: Determine if the Optimal BP reduces the metabolic cost of walking. To accelerate my training and facilitate my development into an independent research scientist, I have brought together mentors with diverse expertise. Dr. Czerniecki is a VA physiatrist and professor of rehabilitation medicine specializing in amputee rehabilitation and gait analysis. Dr. Klute is a VA research career scientist with a vibrant research program focused on prosthetic engineering and amputee mobility. Dr. Thelen is a professor of mechanical and biomechanical engineering and an expert in computational simulations of gait. Dr. Steele is an assistant professor of mechanical engineering and an OpenSim fellow with detailed knowledge or musculoskeletal modeling. Conducting the proposed research project and training program under the guidance of these mentors will allow me to become an expert in the neuromuscular biomechanics of gait, prosthetic engineering, and computational simulations of gait and to establish an independent research career.